1. Field of the Invention
This invention relates to color photographic elements and to a method of preparing the same. More particularly, it relates to photographic elements containing certain yellow coupler dispersions and silver halide having an unexpected increase in photographic activity of the yellow dispersion and also an unexpected increase of the stability of the developed yellow dye to light fade.
2. Description of Related Art
Ballasted photographic dye forming couplers are usually incorporated in photographic systems as a colloidal emulsion, usually called a dispersion in the photographic art. The various methods of preparation of photographic coupler dispersions of prior art are illustrated in FIG. 1. The coupler is added to a high boiling water immiscible solvent (called a permanent solvent) such as tricresyl phosphate, dibutyl phthalate, etc. Some times a low boiling water miscible solvent (called an auxiliary solvent) such as ethylacetate, propanol, methyl-isobutyl ketone, etc. is also added to promote the solubility of less soluble couplers. The mixture is heated to form a true crystal free solution, called the "coupler solution" as indicated in FIG. 1. A second solution of the stabilizing surfactant or surfactants is added to a mixture of gelatin and water and heated to produce a true solution called the "surfactant solution" as indicated in FIG. 1. The surfactant used in the "surfactant solution" is usually an anionic surfactant. The "surfactant solution" and the "coupler solution" are then mixed together with mild stirring to form what is known as a "premix". The premix consists of a crude emulsion of the oil phase (i.e., the "coupler solution") in the aqueous phase (i.e., the "surfactant solution"). The crude "dispersion" is composed of droplets of very large particles, of the order of 5-15 .mu.m in diameter and is as such not usable for high quality photographic products. It is necessary to further reduce the particle size by subsequent high shear milling procedures. The crude "premix" is then further milled in an appropriate high shear milling device. If the coupler solution is composed of only "permanent" solvent, then the dispersion is directly usable in photographic product and such dispersions are usually called "direct dispersions". The particle size of such dispersions are generally broadly distributed with an average diameter between 0.1 to 0.6 .mu.m. As direct dispersions do not contain any auxiliary solvents, no water miscible solvent removal is necessary, resulting in a relatively less expensive dispersion preparation process. Such direct dispersions are then appropriate for price sensitive high volume products such as various types of color papers.
When dispersions are prepared with auxiliary solvents an additional step is necessary to remove the water miscible solvent such that it does not evaporate during the cooling operations to cause excessive evaporation load or create an environmentally hazardous situation. Those dispersions that are treated by some type of an evaporation procedure to remove the auxiliary solvent under a controlled condition are called "evaporated" dispersions. Alternatively, the crude premix can be chill set and noodled by extrusion through orifices and washed by cold water to remove the water soluble auxiliary solvent. Dispersions prepared by this process are usually called "washed" dispersions. Washed dispersions are in general more expensive than evaporated dispersions as they may involve up to 50 hours of tedious washing procedures. Both "washed" and evaporated dispersions lead to dispersion droplets that have broad size distribution with mean diameters ranging between 0.1 to 0.6 .mu.m. "Washed" and "evaporated" dispersions are usually suitable for low volume film products.
U.S. Pat. No. 3,860,425, issued Jun. 14, 1975 to Ono et al teaches the use of a mixture of (1) a nonionic surface active agent containing polyoxypropylene units having a molecular weight greater than 500 and polyoxyethylene units, and at a molar ratio of said polyoxyethylene units to the polyoxypropylene units ranging from 0.1 to 0.6 and (2) an anionic surface active agent having an --OSO.sub.3 M group or an --SO.sub.3 M group, wherein M represents a monovalent cation, and a hydrophobic group in the preparation of a milled dispersion of oleophilic materials, for photographic use. Such dispersions have particle diameters between 0.67 to 0.19 .mu.m. Between about 0.05 to about 0.10 gram of the anionic surfactant and between about 0.02 gram to about 0.10 gram of the nonionic surfactant per gram of the oleophilic dispersed phase of the coupler are used.
U.S. Pat. No. 5,013,640, issued May 7, 1991 to Bagchi et al discloses the use of block oligomeric surfactants comprising hydrophobic polyoxyethylene block (A) and hydrophilic polyoxypropylene block (B) joined in the manner of A-B-A, B-A-B, A-B, (A-B).sub.n .congruent.G.congruent.(B-A).sub.n, a (B-A).sub.n .congruent.G.congruent.(A-B).sub.n, where G is a connector organic moiety and n is between 1 and 3, as melt addenda to reduce viscosity of a microprecipitated dispersion melt in gelatin. The said microprecipitated dispersion being pre-precipitated as a slurry in water before gelatin addition using preferably an anionic surfactant. The particle size of the microprecipitated dispersions have diameters between 0.01 to 0.05 .mu.m.
In cost sensitive high volume products, such as Ektacolor.RTM. Paper or Eastman Color Print.RTM., it is desirable to increase the dye yield of a coupler, as in such a case, it requires less coupler and/or silver, which translates to cost reduction. Further, cyan, magenta and yellow dyes that create photographic images, fade with time when exposed to various ambient lighting conditions such as sunlight, incandescent light or fluorescent light. Most damage is usually done by UV-radiation that may be present in any lighting source. It is therefore desirable to make photographic products, especially photographic paper that is used to display images of both personal and commercial scenes, as stable as possible to fade. There are various means of achieving improved dye stability. One way is to produce couplers that form dye with increased dye stability. Since products such as Ektacolor.RTM. Paper or Eastman Color Print.RTM. are high volume products that are highly price sensitive, it is not always commercially feasible to replace an existing coupler with settled down cost with a new coupler. Photographic papers contain a layer comprising a UV-absorbing compound dispersed in protective layers to absorb the damaging UV-radiation and prevent it from reaching the image dyes. Usually such UV-absorbing compounds have a slight yellow coloration, which when applied in large enough quantities cause the white areas of paper to appear yellow, which is highly undesirable. Therefore, there is a limit to the extent that such UV-absorptive materials can be applied in a photographic product such as paper. U.S. Pat. No. 4,656,125 issued Apr. 7, 1987, to Rinner et al discloses that dye stability can be achieved by the addition of stabilizer compounds to the coupler dispersions. A need to enhance the activity of photographic couplers and enhance the stability of image dyes from fade is desired.
In photographic paper where the image resides on a reflective support light passes through the photographic layers twice for the visualization of the image. In transparency display products such as Duratrans.RTM. light passes through the image only once. Therefore, to produce enough visual density the couplers and the silver halide emulsions are coated at about 70% higher levels on a transparent support. In such case, where the component loading is very high, the bottom yellow layer develops up much more slowly compared to the top cyan and the magenta layers. This requires the use of a longer development time. Therefore, a more active bottom yellow layer in such products with faster development rates will provide a much improved product with shorter development time. Therefore, there is need for the invention of a more active yellow dispersion melt.